With the advent of communication technology, IP telephone services have been started to provide functionality equivalent to the traditional plain old telephone system (POTS).
In such an IP telephone service, the network of the IP telephone service and the network of the plain old telephone system are connected to each other for the purpose of providing communication between the users of the IP telephone service and the users of the plain old telephone system and so forth. Accordingly, if a user uses both services, for example, a call at the telephone number of the IP telephone service assigned to him can be transferred to the telephone number of the plain old telephone system assigned to him.
Also, in the case of an IP telephone service, the user can perform communication at an arbitrary location on a network (IP network), through which IP packets can be transferred, by connecting an arbitrary telephone terminal device (IP telephone terminal) provided with the capability of utilizing the IP telephone service or a personal computer provided with this capability.
However, in order to connect a call to the terminal device and the like at an arbitrary location on the network, the user has to rewrite information such as the IP address of a terminal device in advance registered in a call connection server responsible for connecting calls on the IP network by making use of the redirect mode of SIP (Session Initiation Protocol) which is defined in IETF RFC 2543. In addition, while rewriting such information is permitted typically by a user ID and a password, however, there is a problem that it is not impossible to steal a user ID and a password.
Furthermore, in order to realize the call transfer as described above, there is a problem that the user has to rewrite the information about the forwarding address in a database which is referred to by the call connection server through a Web site and the like every time the terminal device is connected.
The present invention is invented taking into consideration the above problem, and it is an object thereof to provide a communication system, a call connection server, a terminal device and a communication method for automatically transmitting from a terminal device the information for identifying the terminal device and the information for indicating the location of the terminal device on a network, changing the information of the call connection server on the basis of the information as transmitted, and thereby connecting a call at a predetermined telephone number to the terminal device.